The present invention relates to a paste type cadmium anode (negative electrode) for alkaline storage batteries and a method for making the same.
More particularly, the present invention aims at improvement of oxygen gas absorbability in paste type cadmium anodes and improvement of the cycle life of electrode plates by inhibiting distortion of the plates caused by dissolution and precipitation of cadmium.
Recently, paste type cadmium anodes comprising an electrically conductive core material to which a pasty active material is coated are widely used because of simple production process, low production costs and attainment of high energy density.
Different from sintered type cadmium anode, since such paste type cadmium anode has no electrically conductive matrix which holds an active material, growth of metallic cadmium produced at charging of battery occurs in the vicinity of the electrically conductive core material and does not easily reach the surface layer of plate. Therefore, reaction between oxygen gas generated from cathode (positive electrode) at overcharging and metallic cadmium does not proceed efficiently and when such anode is used in a sealed storage battery, there is the problem that internal pressure of battery increases in correspondence to insufficient reaction for vanishment of oxygen gas. Further problem is that as a result of repetition of charge and discharge cycle, dissolution and deposition of cadmium are repeated to cause the distortion of an anode and the reduction of a cycle life of anode.
As an approach to solve these problems, Japanese Patent Kokoku (Post-Exam. Publn.) No. 61-61227 has proposed to apply electrolytic nickel plating to the surface of electrode. However, since nickel plating is directly applied to an electrode, nickel on the surface tends to be uneven and thus sufficient effect cannot be obtained.
Moreover, when a nickel layer is provided on the surface of an electrode, the resulting nickel layer reduces overvoltage for generation of hydrogen and so hydrogen is easily generated from an anode at charging. In case of a sealed type storage battery, hydrogen generated is not consumed in the battery and hence is gradually accumulated in the battery if charging conditions are not proper and this hydrogen gas may actuate an explosion-proof safety device generally employed in the battery to break the sealed system of the battery. In such case, the decrease of electrolyte is brought about to cause reduction in the life of battery. Moreover, in case providing a nickel layer on the surface of an electrode, life characteristics can be improved by proper charging, but examination of a battery whose life has expired reveals the presence of coarse cadmium hydroxide crystals in porous nickel layer formed on the surface of the electrode and it is considered that migration of an electrolyte at charging and discharging at the end of life is hindered by the coarse cadmium hydroxide crystal. Therefore, it is considered that if growth of the cadmium hydroxide in the porous nickel can be prevented, the life characteristics of battery can be further improved.
Furthermore, Japanese Patent Kokai (Laid-Open) No. 60-63875 and U.S. Pat. No. 4,614,696 have proposed to provide an electrically conductive layer comprising carbon powder on the surface of an electrode, but it is difficult to provide a uniform conductive layer as provided by plating. Thus, this also has the problem that sufficient effect cannot be obtained.